The invention relates to the preparation of aldehydes by the hydroformylation process in which an alpha-olefin is hydroformylated with carbon monoxide and hydrogen in the presence of a triorganophosphine-modified rhodium catalyst.
U.S. Pat. No. 3,527,809, entitled “Hydroformylation Process” by R. L. Pruett and J. A. Smith, discloses a significant development in hydroformylation of alpha-olefins to produce aldehydes at high yields at low temperatures and pressures, and with excellent catalyst stability. When the alpha-olefin contains 3 or more carbon atoms, the process produces aldehyde mixtures containing a high normal to iso- (or branched-chain) isomer ratio. The process employs certain rhodium complex compounds to effectively catalyze, under a defined set of variables in the presence of select triorganophosphorus ligands, the hydroformylation of olefins with hydrogen and carbon monoxide. The variables include (1) the rhodium complex catalyst, (2) the olefin feed, (3) the triorganophosphorus ligand and its concentration, (4) a relatively low temperature range, (5) a relatively low total hydrogen and carbon monoxide pressure, and (6) a limitation on the partial pressure exerted by carbon monoxide.
Among the catalysts described in the aforesaid US patent are compounds containing rhodium in complex combination with the carbon monoxide and triarylphosphorus ligands, in particular triarylphosphine ligands exemplified by triphenylphosphine (TPP). A typical active catalytic species is rhodium hydridocarbonyltris (triphenylphosphine) which has the formula RhH(CO) (P(C6H5)3)3. The process uses an excess of the triorganophosphorus ligand.
It is known that despite efforts to prevent it, catalyst activity slowly declines with time. It has been observed that under harsh conditions, the phosphine ligand undergoes side reactions such as aryl exchange between phosphines as well as incorporation of olefin to form alkyl moieties with loss of an aryl group (Abatjoglou, et al., Organometallics, 1984, 3, 923). This alters the make-up of the phosphines present in the system which may form catalyst inhibitors (U.S. Pat. Nos. 4,260,828 and 4,283,304). In aqueous systems utilizing ionic ligands, such an exchange of groups on the phosphorus moiety can result in the loss of the ionic group (Kohlpaintner, et al; Applied Catalysis A 2001, 219), which negatively impacts reactivity and renders the phosphine less ionic which may promote rhodium loss in these biphasic reaction schemes.
U.S. Pat. No. 4,277,627 teaches about several routes of catalyst deactivation including intrinsic deactivation. Operating conditions are specifically stated that minimize the loss of activity with these phosphine-based catalysts. U.S. Pat. No. 4,605,780 teaches that one of the main sources of deactivation of triarylphosphine-based catalysts is the formation of alkyldiphenylphosphines due to alkyl and aryl substituent exchange on the phosphine. U.S. Pat. No. 4,605,780, U.S. Pat. No. 4,710,587 and U.S. Pat. No. 6,946,580 teach off-line processes to reduce the level of these catalyst inhibitors but do not teach how to prevent them. These off-line processes are costly, as production is lost while the catalyst is regenerated.
U.S. Pat. No. 5,237,106, U.S. Pat. No. 5,180,854, and U.S. Pat. No. 4,861,918 teach yet another set of techniques to reactivate Rh-triarylphosphine catalysts using reagents in off-line processing. While effective in recovering much of the catalytic activity, these do not prevent the initial deactivation or future deactivation.
Other examples of catalyst activity loss have been reported for phosphite and polyphosphite-based catalysts. U.S. Pat. No. 6,090,987 discusses the addition of a diene additive to mitigate rhodium deactivation via clustering. U.S. Pat. No. 5,731,472 discusses the use of heterocyclic nitrogen compounds to prevent clustering (as well as the use of acid-removal technologies) in “hydrolyzable” phosphite-based hydroformylation catalysts. U.S. Pat. No. 5,741,942 and U.S. Pat. No. 5,741,944 discuss hydrolysis-based catalyst issues which include the addition of various amine additives which appear to help stabilize these “hydrolyzable” organophosphorus ligands (i.e., phosphites and polyphosphites). However, the type of catalyst deactivation found with these types of phosphite ligands is different than observed with the phosphine-based catalysts which, generally, do not undergo hydrolysis reactions. For example, the hydrolysis-based inhibitors derived from the breakdown of phosphites can lead to rhodium-black (Rh-metal colloids, etc.) and those inhibitors are different from the phosphido-bridged dimers that are derived from phosphine catalysts. For example, U.S. Pat. No. 5,675,041 teaches that Rh-TPP deactivation typically is associated with color formation due to the phosphido-bridged dimers. Phosphite-based activity loss generally does not involve such a color change (e.g., U.S. Pat. No. 5,288,918) or involves Rh loss, possibly with a black/grey appearance.
It would be desirable to have a continuous catalyst activity maintenance process as a means to reduce or prevent the loss of activity in Rh-triorganophosphine hydroformylation catalysts, which process would not involve an off-line removal of the catalyst from the reaction zone and which would maintain high productivity at minimal capital or operating expense.